The brutal monster who murdered a teenager on Valentine’s Day as she held their 5-month-old baby in her arms was sentenced yesterday to a lengthy stint behind bars.

Jesus Urbina, 30 – who slashed the throat of Aura Diaz, 16, with a butcher’s knife on Feb. 14 – stared at the floor yesterday as the judge sentenced him to 21 years to life as part of a plea deal.

“The murder committed by this defendant was particularly atrocious,” Suffolk County Judge Andrew Crecca said yesterday.

“It’s nothing short of pure evil.”

Urbina had been in trouble with the law before the slaying. In 2004, he was deported to his native Honduras for statutory rape after it was discovered he had impregnated Diaz.

After only two months, however, Urbina returned to the United States illegally.

Ignoring an order of protection, he convinced Diaz and her mom, Maria Cardona, 43 – with whom he had a previous relationship – to visit him so he could give his baby Nataly a trinket on the romantic holiday, according

to cops.

At the house, however, he discovered that Diaz had numbers in her cellphone that he didn’t recognize, and exploded in a rage.

While Diaz’s mom looked on, Urbina slashed the teen more than 25 times as she held little Nataly in her arms.